Viscosity increasing agent of polymer-grafted cellulose fibers and method therefor

ABSTRACT

AN AGENT FOR GREATLY INCREASING THE VISCOSITY OF AQUEOUS SYSTEMS WHICH COMPRISES POLYMER-MODIFIED NATURAL CELLULOSE FIBERS WHICH HAVE BEEN WET OR DRY MILLED TO FORM FINE FRAGMENTS OF THE ORIGINAL FIBROUS STRUCTURE. SMALL PERCENTAGES OF ALKALI POLYACRYLATE-MODIFIED CELLULOSE FIBERS MILLED, MECHANICALLY FRAGMENTED OR CRUSHED TO A FINE PARTICLE SIZE HAVE BEEN FOUND EFFECTIVE IN THICKENING AQUEOUS SYSTEMS SUCH AS LATEX BASED PAINTS, HAND AND BODY LOTIONS, SIZING SOLUTIONS AND THE LIKE.

Aug. 8, 1972 VISCOSITY cps.

J W.ADAMS ETAL 3,682,856

VISCOSITY INCREASIN AGENT OF POLYMER-GRAFTED CELLULOSE FIBERS AND METHOD THEREFOR Filed Jan. 29, 1971 EFFECT OF POLYMER/FIBER RATIO ON VISCOSITY OF AQUEOUS SUSPENSIONS OF POLYMER'MODIFIED CELLULOSE FIBERS IO RPM SPlNDLE SPEED &

50 RPM SPINDLE 'SPEED l 1 I 1 I 1 0 I0 I00 SODIUM POLYACRYLATE L 1 2 1 l l I00 90 80 7O 60 50 40 30 20 I0 0 o BLEACHED ASPEN WOOD PULP INVENTORS JAMES WlLLlAM ADAMS AUGUST HENRY TILLOSON AGENT United States Patent @fficCe 3,682,856 Patented Aug. 8, 1972 3,682,856 VISCOSITY INCREASING AGENT F POLYMER- GRAFTED CELLULOSE FIBERS AND METHOD THEREFOR James William Adams and August Henry Tilloson, Schofield, Wis, assiguors to American Can Company, Greenwich, Conn.

lh'led Jan. 29, 1971, Ser. No. 110,812 Int. Cl. C08f 29/50, 45/18 US. Cl. 260-174 GC 12 Claims ABSTRACT OF THE DISCLOSURE SUMMARY OF THE INVENTION This invention relates to a novel synthetic thickening agent which, when added in small amounts to aqueous systems, causes a remarkable increase in the viscosity of the system. More particularly, the invention relates to a water-thickening agent preferably formed by an in situ polymerization of acrylonitrile in and on the fibers of natural cellulose such as that obtained from wood by Well-known processes for the preparation of paper making cellulose pulp fibers. After polymerizing the acrylonitrile in and on such fibers, the resulting polymer-grafted fibers are subjected to an alkaline hydrolysis and the hydrolyzed material, which comprises cellulose fibers having alkali metal polyacrylate bonded thereto, is subjected to a milling operation which partially disintegrates, fragments or crushes the fibrous structure of the cellulose. The resultant product is useful as a thickener in many applications wherein a thickened or viscous aqueous system is desired, including water-based paints and coatings, hand and body lotions and creams, sizing solutions and the like.

In addition to the above method for preparing the alkali polyacrylate-modified fibers, equivalent material may be prepared by polymerizing acrylic acid or acrylate salts in the fibers or by polymerizing methyl acrylate or ethyl acrylate in the fibers and hydrolyzing the product to form alkali metal polyacrylate-modified fibers. These methods for achieving the alkali metal polyacrylatemodified fibers useful in this invention are exemplary and the list is not intended to be exhaustive. The preferred method, however, from the standpoint of ease of operation and economy, involves the polymerization of acrylonitrile in the presence of cellulose fibers followed by an alkaline hydrolysis to the alkali salt of polyacrylic acid chemically bonded to the cellulose fiber structure.

In preparing the fibrous hydrolyzed polymer-grafted cellulose product useful as a thickening agent for aqueous systems, the following procedure is exemplary of the preferred method. Cellulose pulp fibers suitable for use in the process are obtained by a high temperature, high pressure digestion of wood chips by one of the several well-known methods for the preparation of paper-making cellulose pulp fibers, including, particularly, the so-called sulfate or kraft process and the sulfite process for digestion of wood. Cellulose fibers obtained from either hardwoods or softwoods by any of the customary wood pulping processes are satisfactory for use as a raw material base for preparing the material of this invention. Similarly, cellulose fibers from the digestive pulping of straw, bagasse, fiax, bamboo, cotton linters and other similar lignocellulosic sources may be used, although wood cellulose pulp is preferred because of its availability and economy.

The pulp fibers are slurried in water and refluxed with acrylonitrile in the presence of a redox-type polymerization catalyst system such as the well-known ferrous ionhydrogen peroxide catalyst. lolyacrylonitrile is formed both within the walls of the cellulose fibers and on the surface of the fibers and is chemically united to the cellulose molecule in a form of graft polymerization.

When the polymerization is complete, the polymergrafted fibers are removed from the reaction mass and treated with caustic soda or other alkali metal hydroxide at elevated temperature to hydrolyze the nitrile groupings on the polyacrylonitrile to the sodium salt of polyacrylic acid, ammonia being discharged from the vessel as a result of this conversion. After neutralization of the alkaline solution, the reaction product is isolated by filtration, with or without washing of the resulting fibrous mass.

The hydrolyzed, polymer-grafted fibers, added to an aqueous system in the condition described above, will impart only a very slight thickening effect to the aqueous system. It has been found, however, that the fibrous product resulting from the above-described polymerization and hydrolysis has a relatively fragile or fractile structure which can be readily broken down, fragmented or crushed by simple milling procedures to yield a material which is remarkably more effective in the thickening of aqueous systems even when utilized in percentages in the range of 0.1% to about 5.0% by weight of the water present.

It has also been found that the effectiveness of the milled, hydrolyzed polymer-grafted fibers in the thickening of aqueous systems is dependent on the polymer-fiber ratio in the finished product and on several other factors, as will be apparent from the data presented hereinafter.

It is, therefore, an object of the present invention to provide a novel and improved thickening agent for aqueous systems.

It is a further object of this invention to provide an agent which is effective at low concentrations to greatly increase the viscosity of aqueous systems such as waterbased paints, hand and body lotions, fire fighting pastes and the like.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION Further objects will be apparent from a consideration of the following detailed description and drawing which is a graphical representation of the effect of the polymer-tocellulose fiber ratio in the product of this invention on the viscosity of an aqueous system.

The following method may be used for the preparation of the water thickening product of this invention. In this and subsequent examples, quantities of reactants are in parts by weight.

Example 1.--10O parts of bleached aspen kraft wood pulp were slurried in 2,700 parts of deionized water in a reactor equipped with a spiral ribbon agitator. After mixing 15 minutes to break up fiber clumps, 0.3 parts of ferrous ammonium sulfate hexahydrate were added and the slurry pH adjusted to 3.7 by the addition of sulfuric acid of 10% concentration. parts of acrylonitrile, inhibited by the inclusion of 30 parts per million of hydroquinone mono methyl ether, were mixed with the pulp slurry and the reaction mass was heated to 78 C. to reflux the acrylonitrile and purge air from the reactor. After minutes of reflux, heating was discontinued and parts of 10% hydrogen peroxide were added to initiate the on a Brookfield viscometer at 25 C. and 10 r.p.m. spindle speed.

TABLE I.VISCOSITY OF BALL MILLED POLYMER-MODIFIED FIBERS FROM VARIOUS WOOD PULP SOURCES Viscosity in cps. oi 0.33%

suspensions Percent polymer 6 hr. 18 hr.

Sample No. in fibers Type of fiber Unmilled ball milled ball milled 62 Bleached Canadian apsen kraft 1O 8, 100 8, 400

65 Bleached aspen sulfite 570 9, 200 7,000

60 Bleached southern softwood kraft--. 6 8, 500 8,000

62 Bleached Canadian softwood kraft 10 10, 000 9, 300

polymerization of acrylonitrile in and on the cellulose wood pulp fibers. The reaction was exothermic and proceeded under reflux with no further requirement for added heating. After 45 minutes at reflux temperature, the unreacted acrylonitrile was distilled off in a typical stripping operation and the polymer-grafted fibers recovered by filtration and washing. The recovered fibers had 108 parts of polyacrylonitrile intimately chemically bonded to the cellulose, both on the surface thereof and within the structure of the fibers.

Varying amounts of polymer may be structurally united with a given amount of wood pulp fibers by varying the monomer-to-fiber ratio in the reaction mixture. For example, if 400 parts of acrylonitrile are substituted in the above recitation of reactants, the resultant polymer-grafted fibers will retain from 200 to 250 parts of polymer for 100 parts of fiber, and further changes in the ratio of the reactants will be reflected in kind (although not in exact proportion) in the polymer-fiber ratio in the product.

Hydrolysis of the polyacrylonitrile grafted on the cellulose fibers was accomplished by suspending 100 parts of the polymer-grafted fibers in 1,850 parts of water containing 50 parts of sodium hydroxide and the mixture heated at 90 C. for 90 minutes, provision being made for the escape of ammonia gas formed by the hydrolysis reaction. The fibrous, hydrophilic product was washed by settling and decantation.

The washed, wet fibers in aqueous suspension have a minor effect on the viscosity of the aqueous medium, a 1% suspension having a viscosity of only 20 cps. at 25 C. as mesured by a standard Brookfield viscometer at a spindle speed of 10 r.p.m. It has been found, however, that the cellulosic fiber vstructure is so modified by the polymer-grafting process that it becomes remarkably fragile and fractile and is readily fragmented or crushed by various wet or dry milling techniques, whereby the polymermodified. fibers develop a uniquely remarkable thickening effect in water suspension.

For example, agitation of a 0.33% aqueous suspension of the polymer-modified fibers made from aspen kraft fibers and having a polymer-to-fiber ratio of between 1 to 1 and 1.5 to 1 in a Waring Blender for a period of one minute at 25 C. raised the viscosity of the aqueous suspension to a value of more than 6,000 cps., as measured by the Brookfield viscometer, and wet-milling of dilute suspensions of the polymer-modified fibers in a ball mill for varying periods of time results in further very significant increases in viscosity of the aqueous system.

The following Table I includes data indicating the remarkable increase in the viscosity of an aqueous suspension of polymer-modified cellulose fibers as a function of milling time. In this series of tests, polymer-modified fibers prepared from a variety of wood pulp sources and having polymer percentages in the fibers in the range of 60-65% (i.e., about 150-200 parts of polymer to 100 parts of fibers of polymer to 100 parts of fiber) were suspended at a concentration of 0.33% in 1,800 ml. of water and ball milled in a ball mill jar of 4.75 liters capacity together with 4.2 kilograms of ceramic balls of 1.25 centimeter diameter. The jar was rotated at 57 r.p.m. for the periods of time specified in the table and the viscosity measured The results tabulated indicate that viscosity of the suspension is increased greatly by moderate wet milling but that milling beyond the period of six hours gave no further significant viscosity increase. Highest viscosity values were obtained when the starting cellulose fibers were those of Canadian softwood kraft pulp.

Microscopic examination of the wet, unmilled polymermodified fibers shows them to be significantly larger in diameter than the original untreated cellulose fiber. While not wishing to be limited in any respect as to theory, it is apparent that the polymerization and hydrolysis procedure swells and opens up the fibrous structure of the cellulose and loosens or destroys some of the interfiber bonding forces within and between the various layers of fibrils which go to make up the overall structure of an individual cellulose fiber. The polymer-modified fiber structure is thus much more loosely bonded, internally, and is therefore more susceptible than is the original cellulose fiber to defibrillation and fragmentation by me chanical forces such as those encountered in milling procedures. Examination of wet, polymer-modified fibers which have been subjected to a very moderate wet-milling indicates that the original fibers have been fragmented into a multitude of small segments of irregular configuration and which range widely in size as well as shape. Included are partially defibrillated or shredded fragments from which extend ends or sections of individual fibrils and agglome-rates of fibrils and microfibrils. The physical behavior of dispersions of these fragments also suggests the presence of many individual filamental fragments of fibrils, microfibrils and macromolecules which are so small as to be truly colloidal. The macromolecules which are the basic building units of cellulose, as modified by polyacrylate graftings, are in the size range of a few 'angstrom units (10- meters) in diameter and up to about 1 micron (lXlO- meters) in length. The fiber fragment size distribution in the milled aqueous suspensions apparently ranges upwardly from these values to segments or agglomerates as large as 10 to microns (10 to 100x10 meters) in one or more of their dimensions and which may include a mass as great as about 1% of the mass of an average unmilled polymer-modified cellulose fiber, which is normally from 20 to 60 microns in diameter and may have a length as great as one or two millimeters (1 to 2X 10- meters).

In contrast to the viscosity increases achieved by milling the aqueous suspensions of polymer-modified fibers of this invention, neither the original cellulose fibers nor the sodium polyacrylate (NaPA) with which the fibers aire modified demonstrates any significant viscosity increase as a result of milling. This is also true of compounds such as carboxymethylcellulose (CMC) and polyethylene oxide (P-OX) which, together with sodium polyacrylate, are representative of thickeners for aqueous systems which are in common commercial use at the present time.

The following Table H includes viscosity data on milled and unmilled samples of each of the above-mentioned materials and clearly demonstrates the remarkably superior thickening powers of the milled polymer-modified fibers in aqueous systems.

1 Prepared by subjecting the specified pulp to the proccss previously described for preparing polymer-modified fibers but without the incluslon of acrylonitrile, so no polymerization reaction could occur.

2 Prepared by subjecting acrylonitrile to the process previously described for preparing polymer-modified fibers but without the inclusion of any fibers, so that polymerization of acrylonitrile and subsequent hydrolysis occurred as a homopolymerization and hydrolysis of the homopolymer.

3 Bleached Canadian hardwood kraft pulp modified to contain 62% polymer in the manner previously described.

in a determination of the effect of polymer-to-fiber ratio on the thickening power of the polyacrylate-modified cellulose fibers of this invention, viscosity determinations were made on pure cellulose fibers, on sodium polyacrylate and on polymer-modified fibers over polymer-to-fibcr ratios ranging widely within these limits. In each case, the material to be tested was washed and wet ball-milled in aqueous suspension of 0.33% solids contents for six hours and the viscosity of the resulting suspension was determined at 25 C. by a Brookfield Model RVF viscometer at spindle speeds of r.p.m. and 50 r.p.m. The results, as shown in the drawing, clearly indicate that maximum viscosities are developed at a polyme-r-to-fiber ratio of about 150 parts by weight of sodium polyacrylate to 100 parts of cellulose pulp, or a polymer content of about 60% in the polymer-modified fibrous cellulose product. It is evident from the drawing that significant viscosity increases are achieved in very dilute aqueous suspensions (0.33% solids) in all ratios of polymer-to-fiber in the products of this invention. Further testing has shown that the polymer-modified fibers containing from 20 to 90% sodium polyacrylate, and particularly from 50 to 70% polymer, are very readily fractured into high viscosity producing fragments and long periods of milling are not necessary with these materials. Agitation for 60 seconds in a Waring 'Blender develops a viscosity in excess of 6,000 cps. in a 0.33% aqueous suspension of fibers containing 50-60% of the acrylate polymer.

The polymer-to-fiber ratio necessary to obtain the highest viscosities in aqueous suspension varies somewhat with the source and type of wood cellulose fiber employed in producing the polymer-modified fibers. If hardwood (beech or aspen) kraft pulp is used, peak viscosities are obtained at about 55% polymer content, whereas if bleached aspen sulfite pulp is used, highest viscosities are obtained at a polymer content of 6065%.

The drawing also indicates the pseudoplastic character of the aqueous suspensions of the product of this invention since the viscosity readings at a low rate of shear (10 r.p.m. spindle speed) are very much higher than those readings obtained at a substantially higher shear rate (50 r.p.m. spindle speed).

The viscosity of aqueous suspensions of the polymermodified cellulose fibers herein described is dependent on the percent solids composition of the suspension, as well as on the type of cellulose wood pulp fibers utilized, the degree of milling of the polymer-modified fibers and the polymer-to-fiber ratio within the modified fibers. The following Table III shows the variation of viscosity of aqueous suspensions of polymer-modified cellulose fibers as a function of concentration in the aqueous medium. The polymer-modified fibers utilized contained about 52% sodium polyacrylate polymerized on bleached aspen sulfite cellulose fibers (48% The product in each case was wet milled for six hours in a ball mill at the concentration specified and viscosities were determined on a. Brookfield viscometer at spindle speeds of 10 r.p.m. and 50 r.p.m. at 25 C.

TABLE IIL-VISCOSITY OF POLYMER-MODIFIED CELLU- LOSE SUSPENSIONS AS A FUNCTION OF CONCENTRATION The polymer-modified cellulose fibers may also be dried and milled in the dry state to a finely divided pulverulent material which can be added to aqueous systems to increase the viscosity thereof. The dry-milled polymermodified fibers are somewhat less effective thickening agents than the comparable material which has never been subjected to a drying step at elevated temperature. The dry-milled materials therefore must be added in somewhat higher solids concentration in order to achieve a given desired viscosity in an aqueous system. It has been found, however, that if the alkaline hydrolysis step is carried out at very high solids concentration in a reactor having a fluffing or shredding type of mixing action, the resulting moist hydrolysis product is obtained in a crumbly, porous form from which the remaining moisture may be readily removed at only modestly elevated temperature. The dried product obtained in this manner is surprisingly fractile and may be easily pulverized by dry milling techniques to give a fine powder which is extremely efiective at concentrations from less than 1% up to about 5% in thickening aqueous systems.

Example 2.Bleached Canadian hardwood kraft fibers were reacted with acrylonitrile in the manner described in Example 1 to yield polymer-modified fibers having 70% polymer chemically bound in and on the fiber structure. The cellulose fibers having polyacrylonitrile bound thereto were filtered and Washed to yield a moist product of 58% solids content. 3.3 lbs. (dry basis) of this material, having 2.4 lbs. of water associated therewith, was reacted in a ribbon mixer of 1 cubic foot capacity operated at high speed with 1.65 lbs. of caustic soda (solids basis) added as a 30% aqueous solution. After a minute reaction period at C., the moist, porous, crumblike product was dried at 55 C. in an oven. Drying is carried out as rapidly as possible and preferably is complete in six hours or less.

The dry hydrolysis product was then ball milled in a jar containing stainless steel balls of one half inch diameter for 18 hours. A 3% suspension of the resulting powdered product in water had a viscosity of 9,100 cps. as measured at 25 C. with a Brookfield viscometer using a 10 r.p.m. spindle and 2,600 cps., using a 50 r.p.m. spindle and 2,600 cps, using a 50 r.p.m. spindle.

The water thickening properties of the polymer-modified fibers are adversely affected by exposure to elevated temperatures for extended periods of time during the drying process. The following Table IV illustrates the eifect of temperature and time of drying on the viscosity of drymilled polymer-modified fibers. In each case, Canadian hardwood kraft fibers were subjected to a polymerization reaction with acrylonitrile in the manner previously described and the polymer hydrolyzed at a solids concentration of about 60% to form the moist, crumbly, porous hydrolysis product previously described in Example 2, the product containing about 65% polymer and 35% cellulose fiber on a solids basis. Drying was carried out as indicated in the table and the dried product in each case dry-milled in a ball mill for 18 hours. The viscosity of a 3% aqueous suspension was then determined at 25 C. with a Brookfield viscometer.

TABLE IV.EFFECT OF DRYING CONDITIONS ON VISCOSITY Viscosity in cps. of

Oven 3% suspension tempera- Drying ture time in r.p.m. 50 rpm. Sample No. in (3. hours spindle spindle From a consideration of the data in the above Table IV, it is evident that drying the polymer-modified product in a minimum amount of time at only moderately elevated temperatures is preferable in preparing a product of optimum thickening power. Temperatures from about 40- 45 C. to about 75 C. are preferred, although temperatures as high as 125 C. may be used if the drying step is carried out rapidly. Oven time may be minimized by limiting the size of the charge to the oven, smaller amounts of product being dried much more rapidly in an oven of given capacity than larger charges at the same temperature. Temperatures below about 40 C. may also be used but are generally economically unsatisfactory and are therefore not recommended. If desired, other known low temperature drying methods such as solvent replacement methods and freeze drying may be used, but these methods also suffer some economic disadvantages.

In any case, however, the resulting product exhibits a substantial thickening effect on aqueous systems as long as the polymer-modified fibers are milled in either a wet or dry state sufficiently to reduce the particle size of the ground or fragmented fibrous polymer-modified cellulose to less than about 100 microns in its largest dimension, the degree of thickening attainable in a given case being dependent on various processing factors as discussed above as well as on the concentration, polymer-to-fiber ratio, source of the cellulose fibers and the like, as hereinbefore described.

Having now shown and described specific embodiments of the invention, it will be apparent that various modifications may be applied without departing from the spirit thereof and the invention is not intended to be restricted except in accordance with the spirit of the appended claims.

We claim:

1. A method for increasing the viscosity of an aqueous system which comprises adding to said system, in an amount sufficient to effect the desired viscosity increase, a fibrous additive material comprising natural cellulose fibers having chemically bonded therein and thereon by in situ polymerization an alkali salt of polyacrylic acid in a polymer-to-fiber ratio of between 0.25 to 1 and 9 to 1,

said additive material having been subjected to a milling procedure whereby its fibrous structure is fragmented into particles having an average particle size of less than 100 microns in the largest dimension.

2. A method according to claim 1 wherein said additive material is produced by an in situ polymerization of acrylonitrile in the presence of natural cellulose fibers, the polymer-modified fibers so formed subjected to hydrolysis in the presence of an alkali metal hydroxide and the hydrolysis product subjected to a milling procedure sufficient to achieve the desired degree of fragmentation of the polymer-modified fibers.

3. A method according to claim 1 wherein said milling procedure comprises ball-milling of the hydrolyzed polymet-modified fibers in an aqueous suspension.

4. A method according to claim 1 wherein said milling procedure comprises ball-milling of the polymer-modified fibers previously dried rapidly at a temperature of less than about 125 C.

5. A method according to claim 4 wherein said drying step is carried out at a temperature between 40 C. and C.

6. A method according to claim 1 wherein said alkali salt of polyacrylic acid is present in and on said fibers in an amount of between 50% and 70% of the total weight of the polymer-modified fibers.

7. A method for preparing a thickening agent for aqueous systems including the steps of (a) forming polyacrylonitrile in and on natural cellulose fibers by an in situ polymerization of acrylonitrile in the presence of an aqueous suspension of said fibers to form polymermodified fibers having a polymer-to-fiber ratio between about 0.25 to 1 and about 9 to 1 by weight, (b) subjecting the resulting polymer-modified fibers to hydrolysis in the presence of an alkali metal hydroxide whereby polyacrylonitrile is converted to an alkali metal salt of polyacrylic acid and (c) mechanically fragmenting the resulting alkali metal polyacrylate-modified cellulosic fiber product into finely divided fragments, substantially all of said fragments having an individual fragment mass of less than 1% of the mass of the individual unmilled polymer-modified fibers.

8. A method according to claim 7 wherein said mechanical fragmentation is achieved by milling.

9. A thickening agent for aqueous systems comprising natural cellulose fibers having chemically bonded therein and thereon by in situ polymerization an alkali metal polyacrylate, said polymer-modified fibers containing between 20% and by weight of said polyacrylate and having been mechanically milled into fiber fragments having an average particle size of less than about microns in the largest dimension.

10. A thickening agent for aqueous systems comprising insoluble particles of an average particle size of less than 100 microns in the largest dimension prepared from natural cellulose fibers having chemically bonded therein and thereon an alkali metal salt of polyacrylic acid, said polymer being present in and on said fibers in an amount between 20% and 90% of the total weight of the polymer-modified fibers.

11. An agent according to claim 10 wherein said alkali metal salt of polyacrylic acid is present in an amount between 50% and 70% of the total weight of said polymer-modified fibers.

12. A thickened aqueous system including water and between 0.1% and 5.0% based on the Weight of said water of finely divided water-insoluble fragments of natural cellulose fibers having chemically bonded therein and thereon by in situ polymerization an alkali metal salt of polyacrylic acid in a polymer-tofiber ratio between 0.25 to 1 and 9 to l, the average particle size of said fragments being less than 100 microns in the largest dimension.

References Cited UNITED STATES PATENTS 3,107,206 10/1963 Cottet 26017.4 GC 3,325,385 6/1967 Keene et al 260l7.4 GC 3,372,132 3/1968 Cruz 260l7.4 GC 3,425,971 2/19 69 Gugliemelli et a1. 260l7.4 GC

WILLIAM H. SHORT, Primary Examiner E. WOODBERRY, Assistant Examiner 

